A method for modeling the amount of fouling in a heat exchanger comprising at least one shell and at least one tube disposed in said shell to provide heat exchange between the fluids at different temperatures passed through said shell and tube, respectively, characterized by comprising the steps of - assigning an inlet and outlet temperature difference for each of the fluids passed through the tube and the shell; - calculating an iterative outlet temperature for each fluid using the assigned temperature differences and the measured inlet temperatures of the fluids; - determining an expected outlet temperature for each fluid so as to equate it to the calculated iterative outlet temperature; - calculating a mean temperature value for each fluid; - calculating an iterative clean heat transfer coefficient for the heat exchanger using the physical properties under the calculated mean temperatures of the tube and the shell; - calculating a logarithmic mean temperature difference using the inlet temperatures and iterative outlet temperatures of the fluids; - updating the iterative outlet temperatures for each fluid by solving some energy equations of the tube, the shell and of the heat exchanger using the calculated iterative clean heat transfer coefficient, the calculated logarithmic mean temperature difference, the physical parameters of the heat exchanger, the physical parameters of the tube and the shell, and the inlet temperatures of the fluids; - comparing the updated iterative outlet temperatures to the expected outlet temperatures; - returning to the step of determining an expected outlet temperature for each fluid if the difference between the updated iterative outlet temperature and the expected outlet temperature for at least one of the fluids is above a predefined value; - calculating instantaneous values for the heat exchanger using the updated iterative outlet temperatures if the difference between the updated iterative outlet temperature and the expected outlet temperature for each fluid is below said predefined threshold value; and - calculating the amount of fouling in the heat exchanger using the calculated instantaneous values and the calculated iterative clean heat transfer coefficient. - assigning an inlet and outlet temperature difference for each of the fluids passed through the tube and the shell; - calculating an iterative outlet temperature for each fluid using the assigned temperature differences and the measured inlet temperatures of the fluids; - determining an expected outlet temperature for each fluid so as to equate it to the calculated iterative outlet temperature; - calculating a mean temperature value for each fluid; - calculating an iterative clean heat transfer coefficient for the heat exchanger using the physical properties under the calculated mean temperatures of the tube and the shell; - calculating a logarithmic mean temperature difference using the inlet temperatures and iterative outlet temperatures of the fluids; - updating the iterative outlet temperatures for each fluid by solving some energy equations of the tube, the shell and of the heat exchanger using the calculated iterative clean heat transfer coefficient, the calculated logarithmic mean temperature difference, the physical parameters of the heat exchanger, the physical parameters of the tube and the shell, and the inlet temperatures of the fluids; - comparing the updated iterative outlet temperatures to the expected outlet temperatures; - returning to the step of determining an expected outlet temperature for each fluid if the difference between the updated iterative outlet temperature and the expected outlet temperature for at least one of the fluids is above a predefined value; - calculating instantaneous values for the heat exchanger using the updated iterative outlet temperatures if the difference between the updated iterative outlet temperature and the expected outlet temperature for each fluid is below said predefined threshold value; and - calculating the amount of fouling in the heat exchanger using the calculated instantaneous values and the calculated iterative clean heat transfer coefficient.