A detailed study has been conducted of the sorption of ethylene and 1-hexene in linear low density polyethylene (LLDPE) under typical gas phase LLDPE reactor conditions. The recently proposed osmotic ensemble hyperparallel tempering method is used to simulate ethylene and 1-hexene gas sorption in the polymer samples. Simulations are used to parametrize the PC−SAFT equation of state for calculation of gas sorption in amorphous polyethylene. A model for crystallite induced elastic constraints is adopted to modify the equation of state for description of gas sorption in polyethylene below melting point conditions. Single-component ethylene and 1-hexene gas sorption as well as ethylene/1-hexene gas co-sorption are studied in this work. It is observed that simulations provide an effective method to parametrize the PC−SAFT equation of state. In addition, since simulations and PC−SAFT assume a hypothetically amorphous polymer, a quantitative description of elastic effects for gas sorption in semicrystalline poly...