We report the development of a simulation method, with advantages for simulating fluids confined between solid substrates and in equilibrium with bulk fluids. For molecular-dynamics simulations, the isothermal-isobaric constraint method is modified to implement this method. Long-range corrections to the pressure tensor for simple confined systems are also derived and included. Consistent with previous studies employing the grand-canonical ensemble, confined Lennard-Jones and model n-decane fluids investigated with this method show layering induced by the confining surfaces, oscillatory surface-force profiles, and step-like dependencies of the number of confined molecules on surface separation. For a confined Lennard-Jones fluid, increasing the bulk pressure at a fixed temperature enhances layering, increases the effect of surface structure on the surface-force profile, and causes the surface forces to be more repulsive.