This paper describes the development of CAD packages for high pressure polyethylene reactors. The overall goal of the software packages was to develop powerfull, flexible, adaptive design and predictive simulation tools that can follow and predict the operating conditions of a given high pressure polymer reactor in an accurate, prompt and comprehensive way. Their range of use includes the prediction in real time of the molecular properties of polymer produced in high pressure LDPE reactors, the estimation of control moves of key process variables as well as the prediction of the operational and product characteristics of alternative design options. Major points of consideration during the program development were the user friendliness of the input/output and the execution speed enabling its online use as a predictive tool. The functions of the packages have been built around a modular mathematical model of the reactor that includes the energy and material balances based on the leading moments of the Molecular Weight Distribution equations and supported by the physical and molecular properties evaluation equations.