Gas phase polymerization of propylene is one of the most widely accepted and commercially used processes for manufacturing of polypropylene. The present work describes a comprehensive mathematical model for simulating fluidized bed polypropylene reactors. Unlike previously published models, the present model simultaneously predicts both polymer properties as well as particle size distribution. A generalized framework of a dynamic model based on mixing cell approach and detailed polymerization kinetics coupled with population balance model for particle size distribution (PSD) is developed. Need for coupling the reaction engineering model with population balance models is demonstrated. The coupled model was then used to understand influence of operating parameters on polymer properties and particle size distribution. The model is also used to understand the effects of multiple active sites and reaction kinetics on macroscopic variables. The developed framework is useful for simulating multi-monomer, multi-site Ziegler-Natta type olefin fluidized bed polymerization reactors. (C) 2004 Elsevier Ltd. All rights reserved.