A segregation-backmixing model is developed to simulate the dynamic operation of multifeed high-pressure low-density polyethylene (LDPE) autoclaves, calculate the specific initiator consumption (grams of initiator per kilogram of LDPE), and assess the risk of ethylene decomposition under different nonideal mixing and operating scenarios. To describe the nonideal macro- and micromixing phenomena in a LDPE autoclave, a user-specified multizone model representation of the actual reactor is established. One of the key features of the present model is that the continuously fed initiator into a reaction zone can exhibit two distinct states, namely, a "segregated" or a "molecular" one. Simulation results are presented, showing the effects of macro- and micromixing model parameters on the spatial temperature distribution and specific initiator consumption with respect to the initiator feed concentration to a two-compartment reaction zone. Finally, the operational conditions and process faults that can lead to ethylene decomposition in a reaction zone are investigated.