Polymerization processes use two kinds of reactor. One is a continuous reactor and the other is a batch reactor. Both are highly nonlinear and unstable. Thesefore safe operation is important. In a polymerization reactor, raw materials and additives are mixed under specified operating conditions to produce polymer having desired properties and an operator must monitor reactor conditions, especially to avoid runaway reactions. In batch processes, many operation steps are required, and operators have to monitor a number of checkpoints for several processes. Operator support systems, such as diagnosing errors in operation or faults and monitoring process conditions are very helpful for balancing workload and for safe operation. An accurate and widely supported system is developed based on an expert system with a mathematical reactor model to estimate reactor conditions precisely.