With the rise in production of natural gas{,} there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO + H2){,} ethene (C2H4) and acetylene (C2H2). In POX{,} polycyclic aromatic hydrocarbons (PAH) are important undesired byproducts. To improve the productivity of such POX processes{,} it is necessary to have an accurate chemical mechanism for methane-rich combustion including PAH. A new mechanism was created to capture the chemistry from C0 to C12{,} incorporating new information derived from recent quantum chemistry calculations{,} with help from the Reaction Mechanism Generator (RMG) software. For better estimation of kinetics and thermochemistry of aromatic species{,} including reactions through carbene intermediates{,} new reaction families and additional data from quantum chemistry calculations were added to RMG-database. Many of the rate coefficients in the new mechanism are significantly pressure-dependent at POX conditions. The new mechanism was validated against electron-ionization molecular beam mass spectrometry (EI-MBMS) data from a high-temperature flow reactor reported by Kohler et al. In this work quantification of additional species from those experiments is reported including phenylacetylene (C8H6){,} indene (C9H8){,} naphthalene (C10H8) and acenaphthylene (C12H8) at many temperatures for several feed compositions. Comparison of the experimental species concentration data and the new kinetic model is satisfactory; the new mechanism is generally more accurate than other published mechanisms. Moreover{,} because the new mechanism is composed of elementary chemical reaction steps instead of global fitted kinetics{,} pathway analysis of species could be investigated step-by-step to understand PAH formation. For methane-rich combustion{,} the most important routes to key aromatics are propargyl recombination for benzene{,} reactions of the propargyl radical with the phenyl radical for indene{,} and hydrogen abstraction acetylene addition (HACA) for naphthalene.