Computational fluid dynamics (CFD) is used in this study to model low-density polyethylene (LDPE) tubular and autoclave reactors. A polymerization reaction model is developed using the method of moments. The model includes six steps: initiator decomposition, chain initiation, propagation, chain transfer to monomer, disproportionation termination, and combination termination. Coupling of the reaction modeling with the simulation of the flow field is used to predict monomer conversions, polydispersities, radical distributions, and molecular weight distributions. The viscosity of LDPE is defined as a function of the molecular weight and the temperature. Results are obtained for a two-dimensional tubular reactor and a three-dimensional autoclave reactor. The predictions are compared with those of previously published work. Finally, the influence of initiator concentrations and inlet temperatures on monomer conversion and polydispersity is discussed. A sensitivity analyses is used to understand the impact of chemical kinetics on monomer conversion.