In this work, a comprehensive model is developed for the ethylene/1-butene copolymerization in an industrial slurry polymerization reactor for linear low-density polyethylene synthesis. The model is able to describe the dynamic evolution of the molecular weight averages, comonomer content, particle size averages, melt index, and density of the final polymer resin and extends modeling results available in the open literature. A new modeling approach is used to describe the evolution of particle sizes, which is based on the definition of a joint distribution of mass and catalyst concentration of solid polymer particles. It is shown that the model successfully describes the operation of an industrial slurry polymerization reactor. For this reason, the model is used to analyze how sensitive the final polymer properties are to variations of the feed conditions and to development of segregated mixing zones inside the reactor vessel. It is shown that the ethylene feed flow rate is the most influential process variable and that the existence of very small segregated reactor zones can lead to very serious operation problems, such as particle agglomeration.