An off-line dynamic optimization procedure is employed to optimize the transition between different grades of linear low density polyethylene in a fluidized-bed reactor. This type of reactor is frequently operated under condensed mode, which consists of injecting induced 2 2 condensing agents (ICA) to absorb part of the reaction heat. However, the presence of ICA affects the solubility of monomers in the polymer, so it is important to account for this effect in a grade transition optimization strategy. A kinetic model is combined with a thermodynamic model based on the Sanchez-Lacombe equation of state to describe the grade transitions. Simplified correlations are then suggested to predict the impact of ICA on ethylene and comonomer solubility in a quaternary system. The results highlight the importance of the thermodynamic model during grade transition.