1. A method for metering an ammonia-releasing reducing agent into an exhaust gas stream of an internal combustion engine operating with excess air in an exhaust gas post-treatment system having at least one SCR catalyst arranged in the exhaust gas stream downstream of a metering device, comprising: metering the ammonia-releasing reducing agent into the exhaust gas stream by the metering device that is controlled by a control unit assigned to the exhaust gas post-treatment system, wherein the metered quantity of reducing agent is determined by the control unit based on a model that covers a plurality of operating points of one of the internal combustion engine and the exhaust gas post-treatment system as a function of at least one operating parameter of the one of the internal combustion engine and the exhaust gas post-treatment system evaluated by the control unit, wherein an instantaneous value of the at least one operating parameter determines an associated one of the plurality of operating points of the one of the internal combustion engine and the exhaust gas post-treatment system; comparing by the control unit, during operation of the internal combustion engine, a first stored value, which is proportional to one of a nominal emission and a nominal conversion determined for the one of the internal combustion engine and the exhaust gas post-treatment system, with a second value, metrologically obtained, that is proportional to one of an actual emission and an actual conversion; determining a deviation between the first and second value; determining a correction value for the metered quantity based at least in part on the determined deviation; modifying the model by the correction value for the metered quantity for a subsequent metering processes, the modified model being used until another deviation between a value which has been stored for the associated operating point of the one of the internal combustion engine and the exhaust gas post-treatment system that is proportional to the nominal emission or to the nominal conversion and a value which has been newly determined metrologically by the control unit and which is proportional to the actual emission or to the actual conversion; determining at least one of: the one of the actual emission and the actual conversion by one of adding and integrating a measurement value of at least one of an NOx sensor, an NH 3 sensor, an N 2 O sensor, an HNCO sensor, and a lambda sensor installed downstream of the SCR catalyst, and the actual conversion by one of adding and integrating a measurement value of at least one of the NOx sensor, the NH 3 sensor, the N 2 O sensor, the HNCO sensor, and the lambda sensor and a raw emission upstream of the SCR catalyst, one of for a predefined time, until a predefined emission quantity or amount of work is reached, and until at least one of the said at least one operating parameter reaches a predefined value by addition or integration; monitoring the results of the determining to further determine whether or not the at least one operating parameter lies within one of at least two different value ranges, wherein the value ranges are determined by stored values; at least one of: discarding the result of the addition or integration when the at least one operating parameter has left the value range during the addition or integration process; and storing the sum obtained by addition or integration on an interim basis and then, when the control unit determines that the at least one operating parameter has returned to the value range it had previously left, resuming the addition or integration process until the one of the predefined time for addition or integration has passed, until the predefined emission quantity or amount of work is reached, and until the at least one operating parameter has reached the predefined value by addition or integration; using, by the control unit, the sum obtained by addition or integration over the predefined time or until the predefined emission quantity or amount of predefined work is reached or a value proportional to that sum as the actual emission or actual conversion, for comparison with the nominal emission or nominal conversion obtained from the stored values, to determine the correction value for the metered quantity; and one of: modifying the metered quantity from the model by the correction value determined for the associated value range of the at least one operating parameter, when the value of the at least one operating parameter determined by the control unit lies in this value range; and determining, by the control unit, as a function of the value of the at least one operating parameter determined by the control unit, by interpolation, a correction value from correction values which were determined for values of the at least one operating parameter immediately adjacent to the value and modifying the metered quantity from the model by this determined correction value.