In grand canonical ensemble Monte Carlo (GCEMC) simulations, we investigated the microscopic structure of liquid-crystalline films confined between two plane parallel solid surfaces (i.e. walls) consisting of N-s discrete, rigidly fixed atoms distributed across the plane of a wall according to the (100) structure of the face-centered cubic lattice. Parameters of the film-wall interaction potential are chosen such that a homeotropic alignment of film molecules is favored. In the simulations the thermodynamic state of the film is determined by the temperature T, the chemical potential mu, the distance between the walls s(z), and the film-wall interfacial area A. Thermodynamic states of the film are chosen such that a corresponding bulk liquid crystal is nematic. These films are referred to as 'nematic' to emphasize the physical nature of the bulk phase in thermodynamic equilibrium with the film. To simulate nematic phases in GCEMC we modified the classic Gay-Berne potential for the interaction between a pair of film molecules, so that the isotropic-nematic phase transition in the bulk occurs at sufficiently low densities. Variations of the microscopic structure with increasing st are correlated with the normal component of the stress tensor T-zz (s(z)). Our results show that molecules in inner portions of the film undergo a reorganization from an originally planar orientation of their symmetry axes to a perpendicular one with respect to the plane of a wall. This orientational change is manifested as a periodic sequence of shoulders in T-zz(s(z)) where the periodicity length Delta s(z) is close to the larger diameter of the ellipsoidal film molecules.