A dynamic mathematical model for liquid-phase polymerization in loop reactors was developed and implemented in language C using S-functions in a MATLAB/SIMULINK environment. It is based on a nonideal continuous stirred-tank reactor (CSTR) model capable of dealing with multisite copolymerization of olefins. The kinetic scheme includes a specific mechanism for hydrogen effect on the rate of polymerization observed on both laboratory experiments and industrial plant trials. A nonideality due to polymer segregation at the reactor output was inserted into the model to better predict reactor slurry density. Polymer moment balances were used to compute resin properties, such as average molecular weights, polydispersity, and melt flow index. Dynamic data from an industrial polypropylene plant were used for parameter estimation and model validation.