A nitrogen-rich inert gas mixture is produced by the partial oxidation of a hydrocarbonaceous feed containing substantially no metals nor noncombustible materials with air in a free-flow, unpacked, refractory-lined gas generator at a temperature in the range of about 1300.degree. to 3000.degree. F. and a pressure in the range of about 1 to 250 atmospheres. The product gas will comprise a mixture of nitrogen, argon and carbon dioxide and may contain small amounts of hydrogen and carbon monoxide, depending on the O/C atomic ratio selected. The atomic ratio of free oxygen in said air to carbon in said hydrocarbonaceous fuel is in the range of about 1.7 to stoichiometric, or slightly less than stoichiometric. By operating at this level of O/C atomic ratio, the H.sub.2 + CO content of the product gas may be minimized or deleted, substantially all of the particulate carbon may be oxidized, substantially no NO.sub.x is produced, and the product gas contains substantially no free oxygen. Further, the sensible heat recovered from the product gas may be used to manufacture by-product high pressure steam for export. The nitrogen-rich product gas may be used for oil formation flooding, or as a pressurizing or blanketing gas. Costly gas compressors may be avoided since the product gas may be produced at or above use pressure.