This study considers the development of optimization models for grade transition of polyethylene solution polymerization processes. A detailed mathematical model is developed to capture the dynamics of the solution polymerization process. This includes time delay models for vapor and liquid recycle streams as well as a reduced, yet accurate, vapor-liquid equilibrium (VLE) model derived from rigorous VLE calculations. Simultaneous dynamic optimization approach is applied to solve the optimization problem to reduce off-spec production time and transition time. Two optimization formulations, single stage and multistage, are developed to deal with single-value target and specification bands of product properties, respectively. The results show significant reductions in grade transition time and off-spec production time. In addition, the multistage formulation designed for problems with specification bands outperforms its single stage counterpart. It minimizes transition time and off-spec production directly, and leads to higher performance control profiles. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 1126-1142, 2016