Abstract <p>At present, an effective method to protect pipelines from harmful external forces is the polymeric coating of the inner wall of a steel pipeline, which protects steel from the effect of a chemically aggressive medium. Methane diffusion through the polyethylene layer used as the coating for oil steel pipelines is modeled by the molecular dynamics method. The structure of polyethylene through which methane gas passes upon the external pressure action is studied. Methane molecules are shown to penetrate through the polyethylene structure, however, their further penetration is limited by polyethylene macromolecules upon the external pressure action. Diffusion of methane molecules is analyzed together with their average and maximum penetration depths through polyethylene at different temperatures. The results obtained enable the prediction of the minimum thickness of the polyethylene coating of a steel pipe, which is enough to protect the wall from the harmful methane effect.</p>