Novel Monte Carlo methods are used to simulate the excess chemical potential of commercial hindered-amine light stabilizers (HALS) in polyethylene. The solubility change incurred by increasing the size of the pendant tail of HALS molecules by a single methylene segment is estimated using simulations. Our results indicate that HALS molecules undergo a conformational transition at intermediate pendant-tail chain lengths. Our results also indicate that, above the boiling temperature of a particular stabilizer HALS, a segmental chain length increase raises its solubility in polyethylene. Similar phenomena are observed for small alkanes in polyethylene both theoretically and experimentally. The relative solubility of HALS of different lengths appears to increase with decreasing temperature. Results of molecular dynamics simulations yield diffusion coefficients that are consistent with available experimental data.