To meet customers' various and complicated demands, polyolefin plants are forced to have frequent grade transitions that cause the production of a large amount of off-specification product. To reduce the amount of off-specification product and to maintain property uniformity within the specified grade, a virtual on-line analyzer (VOA) is necessary that calculates polymer properties from values of process variables that can be measured on-line such as temperature, pressure, gas composition, and so on. Most research has focused on the prediction of properties of the polymer in the reactor so far. However, in bimodal high-density polyethylene (HDPE) processes, there are significant differences between the properties of the polymer from the reactor and those of the ultimate form of the polymer, i.e., pellets. In this study, a VOA that calculates pellet properties such as the melt index (MI), density, and melt flow rate ratio (MFRR) is developed for bimodal HDPE processes. To estimate the MI and MFRR of the pellet, the concept of mode properties is introduced. The pellet properties are calculated using a combination of the mode properties of the polymer that was produced in each reactor and other operating conditions. An iterative technique is also used to develop a density model to overcome the lack of measurements. Further, by considering the catalyst deactivation mechanisms, mixed catalyst systems can be modeled with the proposed VOA system. An off-line Excel-based program is also developed for use in product design prior to commercial production trials to determine whether a certain new grade can be produced. With all of these components, the proposed VOA system has contributed to effective plant operation.