1. A method of controlling the feeding of a reducing agent, such as urea or ammonia, to a selective catalytic reduction reactor which is operative for removing NOx from a process gas of a process plant, such as a combustion plant or an incineration plant, said method of controlling the feeding of the reducing agent comprising the steps of: estimating a present value of at least one predetermined parameter which is indicative of the amount of NOx that needs to be removed from said process gas in the selective catalytic reduction reactor, estimating a rate of increase of said at least one predetermined parameter, increasing, when said estimated rate of increase of said at least one predetermined parameter is positive, the amount being supplied of said reducing agent to a higher value than would be the case if said at least one predetermined parameter were to be kept constant at the estimated present value thereof.