Computational fluid dynamics (CFD) was employed to study decomposition of ethylene within a low-density polyethylene 4-zone reactor to reduce the accumulation of hot unmixed material. An accurate model can predict conditions resulting in thermal runaway, which can cause unplanned shutdowns. Improving the reactor design allows for longer run times and a higher degree of catalyst conversion. The CFD model employed reaction kinetics, PID-automated catalyst injection systems, and a rotating stirrer shaft to rigorously model the conceptual reactor. Early models identified zones which were not well-mixed, and wall-mounted baffles were modeled to improve mixing. Later, "hotspots" were defined as contiguous volumes greater than one cubic inch and above 300°C. The current model revealed that hotspots were concentrated around the stirrer shaft, and characteristics of those hot spots were elucidated.