The propagation and interaction of the optical radiations are analyzed using the theory of a highly multimodal nonlinear guided wave methodology instead of plane wave optics for explaining the optical rotation quasi-phase-matching technique based on total-internal-reflection. Thereby, the yielding second harmonic from a yttrium oxide-coated rectangular slab of magnesium oxide-doped lithium niobate crystal shows very high efficiency. The conversion efficiency of 2.7% has been observed by performing the computer-aided simulation and corresponds to a second harmonic wavelength of 532 nm after considering the surface roughness and absorption.