The production of high-density polyethylene involves frequent grade transitions due to increased diversity among the products demanded by consumers. In this study, staged property models are developed to predict the changes in polymer properties during continuous, discontinuous, and semi-continuous grade transitions. At the outset, the operation states of the reactors are divided into three stages according to their distinct dynamics. Polymer property models are subsequently formulated for each stage based on empirical models. Model parameters are updated on a real-time basis according to the change of the partial properties of polymers in the reactors. The effectiveness of the proposed model is demonstrated via application to an industrial slurry high-density polyethylene process.