Most polyolefin products are prepared by solid-catalyzed olefin polymerization in continuous processes. The natural presence of reactor residence-time distributions implies a distribution of polymer properties in the final product, which can have a significant effect on the quality of the resin. In this work we apply a population balance approach for the prediction of important polymer properties such as molecular-weight and copolymer composition distributions. The modeling concepts are illustrated with examples of bimodal polypropylene produced in two stirred-bed reactors in series and linear low-density polyethylene. For linear low-density polyethylene, comparisons between a well-mixed reactor and a fluidized bed with particle-size-selection effects are shown. It is demonstrated that residence-time distribution effects play a significant role on the establishment of polymer architecture properties such as molecular-weight and average copolymer composition distributions.