We present the development of both steady-state and dynamic models for a slurry HDPE process using fundamental chemical engineering principles and advanced software tools, Polymers Plus and Aspen Dynamics. The discussion includes thermodynamic properties, phase equilibrium, reaction kinetics, polymer properties, and other modeling issues. We characterize a Ziegler-Natta catalyst by assuming the existence of multiple catalyst site types and deconvoluting data from gel permeation chromatography to determine the most probable chain-length distributions and relative amounts of polymer produced at each site type. We validate the model using plant data from two large-scale commercial slurry HDPE processes. Significantly, the model contains a single set of kinetic and thermodynamic parameters that accurately predicts the polymer production rate, molecular weight, polydispersity index, and composition for several product grades. We illustrate the utility of the dynamic model by simulating a grade change. Finally, we propose a process retrofit that permits an increase in the HDPE production rate of up to 20% while maintaining the product quality.