A mathematical model for steady-state slurry and bulk propylene polymerizations is developed and implemented. The model is based on the detailed formulation of mass balances for all chemical species present in the reaction environment, of energy balances for the reactor mixture and cooling water and of momentum balances. The model allows the computation of mass and energy inventories and the prediction of polymer molecular structure and morphology, including the molecular weight distribution, the chain composition distribution, the particle size distribution and the stereo-block distribution. Besides, end-use properties, such as the melting-flow index, the impact strength and the flexural modulus, are described through empirical models that depend on the molecular and morphological proper-ties of the polymer resins, as described by Latado et al. (Polymer Testing 20(4) (2001) 419-439). The model also allows the simulation of the extrusion operation and is validated with actual operation data for two different polymerization plants, using different Ziegler-Natta catalysts.