This paper describes the development of a comprehensive model for the continuous gas-phase synthesis of polypropylene using stirred-bed reactors. The model considers the important issues of physical property and thermodynamic model selections, polymer properties, catalyst characterization, and reactor residence time, in addition to the traditional Ziegler-Natta polymerization kinetics. Model development involves fundamental chemical engineering principles and advanced software tools, Polymers Plus and Aspen Dynamics. We characterize a Ziegler-Natta catalyst by assuming the existence of multiple catalyst site types. The model contains a single set of kinetic and thermodynamic parameters that accurately predicts the polymer production rate, molecular weight, polydispersity index, and composition for both homopolymer and impact copolymer product grades from a large-scale commercial process. We demonstrate the application of our dynamic model and process control by comparing grade-transition strategies.