Continuous-flow experiments in an empty aluminum oxide tube for the investigation of the homogeneous gas-phase reactions occurring at conditions typical for the partial oxidation of methane to synthesis gas were carried out at pressures from 300 to 800 kPa, temperatures from 1030 to 1300 K, inlet molar ratios of CH4/O-2 from 2.1 to 3.1, and residence times from 0.09 to 0.38. The oxygen conversion varied from 3% to 43%. A detailed kinetic model based on a free-radical mechanism has been developed, which allows the adequate calculation of the feed conversions and product selectivities. The model includes pressure falloff effects occurring with the unimolecular reactions. The model reveals important features of the complex reaction network with the emphasis on the ignition stage at oxygen conversions between 0% and 5%. The occurrence of both isothermal ignition and thermal ignition can be distinguished in simulations with the model. The model was also used for estimating the ignition delays as a function of the process conditions.