In this white paper, we have developed a numerical technique to accurately model the experimental near-field amplitude and phase spectra in near-field infrared nano-spectroscopy. Our numerical method using Altair Feko is especially useful for describing surface phonon-polaritons. Also, extension of our numerical technique to materials with anisotropic dielectric function and/or heterogeneous structure is possible in the future. This method is efficient to elucidate fundamental physical phenomena measured in near-field infrared nano-spectroscopy experiments on novel, complex materials.