Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anisotropy of the perpendicular, P-N(z), and parallel components, P-T(z), of the pressure tensor the surface tension of the system is calculated for a wide range of temperature and for various film thicknesses. Three methods of determining the pressure tensor are compared: the method of Irving and Kirkwood (IK), an approximation thereof (IK1), and the method of Harasima (H). The IK- and the H-methods differ in the expression for P-T(z) (z denotes the distance from the wall), but yield the same formula for the normal component P-N(z). When evaluated by molecular dynamics (or Monte Carlo)-simulations P-N(z) is constant, as required by mechanical stability. Contrary to that, the IK1-method leads to strong oscillations of P-N(z). However, all methods give the same expression for the total pressure when integrated over the whole system, and thus the same surface tension, whereas the so-called surface of tension, z(s), depends on the applied method. The difference is small for the IK- and H-methods, while the IK1-method leads to values that are in conflict with the interpretation of z(s) as the effective position of the interface.