Many polyolefins are produced by catalyzed olefin polymerization over solid catalysts. A variety of processes are currently in use including loop, continuous-stirred tank, horizontal stirred bed and fluidized-bed technologies. In each of these continuous processes, the reactor residence time distribution is unique and affects the resulting distribution of polymer properties. In this paper we present a population balance modeling approach for modeling multistage olefin polymerization processes using catalyst residence time as the main coordinate. The catalyst may possess a broad particle size distribution and contain multiple active sites with a comprehensive kinetic scheme involving multiple monomers. Diffusion limitations during reaction may also be a consideration. A variety of reactor systems can be modeled each with unique residence time distributions and having different internal and external residence time distribution. In addition, the modeling allows the consideration of particle size selection effects within the process. These effects are implemented in a model of a fluidized-bed reactor. The population balance model is illustrated with examples of propylene polymerization in the four major commercial processes listed above, with a high activity TiCl4/MgCl2 catalyst. Comparisons of total production capacity, catalyst yield distribution, polymer particle size and porosity distribution are made. It is found that significant differences exist between the different industrial processes. It is also found that the performance of the fluidized-bed reactor is highly sensitive to fluidization conditions, particle size selection effects and the catalyst particle size distribution. Differences in product quality are discussed and issues such as fines generation and particle sticking are addressed.