1. A method for operating an industrial plant, comprising: having a rule-based expert system that comprises a set of rules for evaluating the operation of a process unit in the industrial plant; having a model-based expert system that comprises a mathematical model for evaluating the operation of the process unit in the industrial plant; applying the model-based expert system to evaluate the operation of the process unit using data on the operating conditions of the process unit; obtaining a result from the model-based expert system; applying the rule-based expert system to identify a possible abnormality in the operation of the process unit using: (a) data on the operating conditions of the process unit, and (b) the result from the model-based expert system; sending a message regarding an abnormality to an operator interface; suppressing a message from the model-based expert system, wherein the suppressed message relates to the abnormality; and adjusting the operation of the process unit based on the message relating to the abnormality.