1. In a digital processor, a method for modeling a polymerization process carried out in a polymer manufacturing system formed of plural reactors and ancillary equipment, the method comprising the steps of: receiving indications of reactor type and polymers involved in the polymerization process, each polymer having at least one molecular property; determining instantaneous property measures of the molecular properties of the polymers; generating and tracking respective instantaneous property distributions of the molecular properties from the determined instantaneous property measures; from the instantaneous property distributions, calculating full distributions of the molecular properties at any location in the manufacturing system; using the calculated full property distributions, forming and reporting a model of the polymerization process such that the polymer manufacturing system is modeled; and the method being capable of modeling both linear and non-linear polymerization processes.