A three-dimensional (3D) model of a methane-air counter-flow, non-premixed flame with a global reaction step for methane oxidation was developed using computational fluid dynamics (CFD) simulation. A specific computational domain, and relevant thermodynamic and transport data calculated by the chemical kinetic code CHEMKIN, were incorporated into the model. The model was validated by comparing predictions with the spontaneous Raman scattered profiles of major combustion species reported in the literature. The model was employed to carefully examine the self-similarity assumptions normally invoked in simulating counter-flow non-premixed flames. It was found that while most assumptions were strictly satisfied within the jet region for the case of plug flow boundary conditions (B.C.) along the central axis, for the quadratic boundary condition case (corresponding to uniform plug-flow), assumptions were only approximately valid within the jet region. Also, the influence of shroud gas was examined by setting the surrounding gas as air and increasing the shroud gas through widening the shroud gas gap while maintaining a constant shroud gas velocity. Calculations revealed that, the resulting flames for shroud gas gaps greater than half of the jet radius, were totally insulated from mixing with the ambient air. The effect of buoyancy on the flame structures was also studied by comparing contours of the combustion products, temperature and turbulent properties.