An osmotic ensemble hyperparallel tempering technique has been developed to study the solubility of ethylene in amorphous linear low-density polyethylene of different chain architectures. The NERD united-atom force field (Nath, Escobedo, and de Pablo revised united-atom force field) is used in all simulations. We have investigated the effect of polyethylene chain length and branching on ethylene solubility. In this study, we have considered short-chain branching of amorphous linear low-density ethylene-1-hexene copolymers under typical polymerization reactor conditions. It is observed that, in the polymer, ethylene prefers to reside in the vicinity of polymer chain ends. This clustering causes a decrease in ethylene solubility with polymer chain length. When short-chain branches are introduced to a linear polymer chain, however, the chain-end clustering effect is counteracted by a higher density, thereby leading to an ethylene solubility almost identical to that in the linear polymer.