We report on an application of a three-dimensional pseudospectral time-domain algorithm that solves with accuracy the nonlinear Maxwell’s equations so as to predict second-harmonic generation in lithium niobate ridge-type waveguides with high index contrast. Characteristics of the nonlinear process such as conversion efficiency and impact of the multimode character of the waveguide are investigated as a function of the waveguide geometry in uniformly and periodically poled media.