A detailed mathematical model is developed to describe the dynamics of continuous soluble Ziegler-Natta ethylene polymerizations in reactor trains composed of series of both stirred tank and tubular reactors. The model comprises a detailed description of the reaction mechanism and the mass, energy, and momentum balance equations for each reactor vessel, which are then used to allow the prediction of final polymer properties (average polymer molecular weights, polymer density, melt flow index, etc.) and process performance (polymer productivity, head losses, energy consumption, etc.). Tubular reactors are assumed to behave as ideal plug flow reactors, and the method of characteristics is used to solve the set of algebraic-partial differential equations that describe the process. Plant data are used to validate the model, which is shown to describe very well the operation of actual industrial reactors.