The present work concerns modelling of semi-crystalline polymers, such as polyethylene. Semi-crystalline polymers constitute a material group with an increasing industrial importance. Constitutive response (stress-strain relation) as well as fracture properties are of great importance for this class of materials. We propose an anisotropic plasticity model for semi-crystalline polymers, formulated in the reference configuration (Lagrangian formulation). In addition to plasticity, the model accounts for effects of viscoelasticity. The model is evaluated using uniaxial tensile tests, where polyethylene sheets are tested. The model is able to predict the hardening and rate-dependency observed in the experiments very well. Numerical examples are also presented, where the model is applied to a few different geometries