High-pressure ethylene polymerization has been studied for a single-zone and two-zone autoclave reactor systems through modeling and simulation. For a single-zone autoclave reactor, the effect of imperfat mixing in the reactor is analyzed using both an imperfect mixing model and a perfect mixing model with temperature variant initiator efficiency factor. In both models, a detailed polymerization kinetic model is incorporated into the reactor models for the calculation of reactor performance variables and polymer properties such as molecular weight averages, and short-chain and long-chain branching frequencies. The simulation results of these models show that the steady-state reactor temperature is little affected by imperfect mixing for the volume fraction of segregated zones less than 10% of the total reactor volume. As the degree of imperfect mixing increases, an increased amount of initiator is required and thus the specific initiator consumption rate increases. However, the imperfect mixing has only a minor effect on the polymer properties. The effect of using a binary mixture of initiators on the reactor performance is also examined for a two-stage autoclave reactor. It is shown that the reactor performance may vary significantly with the feed composition of the initiator mixture.