Abstract: A fuel-gas blending system receives low-pressure tank vapors and high-pressure flash gases from an oil production facility, boosts the pressure of the tank vapors, and blends the tank vapors and high-pressure gases together to supply fuel gas at a pressure and quality required by an onsite fuel-gas-powered generator. The quality of the supplied fuel gas is maintained by controlling the proportion of a high-pressure gas, such as separator gas, in the blend while the volumetric flow rates of the various gases vary in response to the real-time demands of the generator. The system operates in one of multiple modes in order to maximize the use of tank vapors. In one mode, all the gases pass through a low-pressure blower. In another mode, only the tank vapors pass through the blower, and the high-pressure gases are blended with tank vapors downstream of the blower.