A stationary theoretical model giving an asymptotic description of the propagation of nonlinear electromagnetic waves in an inhomogeneous plasma is presented. The plasma, considered within the hydrodynamic approximation and assumed to be cold, collisionless, unmagnetized and electroneutral, is described in terms of a nonlinear dielectric function giving the effect of the Miller ponderomotive force. In an infinitely extended slab geometry, a variable transverse density gradient, assumed to be parabolic, is considered in the case when a monochromatic electromagnetic wave is incident upon the slab at an arbitrary angle. The transverse structure of the wave electric field is determined along with the perturbed plasma density profile. The nonlinear wave equation with slowly varying coefficients derived using this model is solved in first and second approximations using an asymptotic multiple-space-scale method due to Bogolyoubov and Mitropolskii. The critical conditions for resonant electrostatic absorption and mode conversion are derived and discussed within these approximations, and some reference numerical calculations are performed.