A dynamic process model has been derived for the time dependence of cumulative polymer property distributions in well-mixed continuous polymerization reactors using instantaneous property methods and linear mass-weighted mixing rules. An analytical solution is obtained for the cumulative distribution under transient conditions: the resulting convolution integral can be evaluated in a two-step process using the output from a dynamic reactor simulation package. Alternatively, the process model derived for cumulative distributions can be incorporated into reactor simulation code and the distributions simulated directly. Examples of the impact of catalyst kinetics, reactor operating policies, and process upsets on the molecular weight distribution of bimodal high-molecular-weight polyethylene produced in well-mixed, stirred-bed, gas-phase reactors are simulated for a binary transition-metal catalyst system using the convolution integral approach and output from POLYRED.