We present a computational technique for modeling the dynamics of polydisperse polymeric systems undergoing reaction-induced phase separation. In particular, we demonstrate how the Cahn-Hilliard method, incorporating the Flory-Huggins free energy, can be used to simulate the evolution of a polydisperse-monodisperse polymer blend. To computationally model highly polydisperse polymeric systems, with constituents ranging from monomers to linear chains comprised of 10(5) segments, we suitably discretize the molecular weight distribution. We demonstrate how our reduced component scheme does not affect the general polymer blend dynamics, before using our approach to elucidate the effects of polydispersity on the dynamics of such systems. In this manner, the complex dynamics of phase separation between polymers of different species and the interdiffusion between polymers of different lengths are accurately captured. Furthermore, we couple our model of polymer blend dynamics with Flory-Stockmayer reaction kinetics and simulate the dynamics of reaction-induced phase separation in polymer blends. We demonstrate that the behavior of such systems is sensitive not only to the complex interplay between enthalpy and entropy but also to entanglement effects between polymers of increasing lengths.