A comprehensive mathematical model is developed to simulate the ethylene polymerization in high-pressure autoclave reactors. Two generalised macromixing models, namely the external recycle and the backflow model are established to describe the complex mixing patterns occurring in multizone, multifeed low density polyethylene (LDPE) autoclaves. According to these models, each zone in the reactor is divided into a sequence of perfectly mixed vessels, called segments. To represent the kinetics of ethylene polymerization a general reaction mechanism is considered and the method of moments is employed to calculate the molecular weight developments. A new technique the so-called ’numerical fractionation’ is applied to predict the formation of gel and the number chain length distribution in LDPE autoclaves. Finally, a detailed stability analysis is carried out for an LDPE autoclave to identify the number of multiple steady-states.