Much effort has gone into the study of silica and its material properties due to its importance in long distance communication. Especially, the propagation of optical pulses in single-mode fibers and their interplay with nonlinear effects have been modeled with good accuracy. This thesis first presents a derivation of a frequency-domain Generalized Nonlinear Shrödinger Equation that has been used in the simulation of nonlinear phenomena relating to ultrashort pulses in single-mode fibers. It then develops an even more general approach and shows in detail how it can be used to successfully model the results of an experiment in which high-efficiency intermodal four-wave mixing in a higher-order-mode (or few-mode) fiber was observed. A formulation that can model interactions between vector-polarized modes is also covered.