Method and system for blending biogas with conventional fuel in which the fuel blend is automatically adjusted for lower biogas flows and methane concentrations by introducing higher concentrations of conventional fuels. The system is able to automatically adjust the fuel blend, thereby eliminating the requirement for manual intervention, and producing a variable blended biogas that can be utilized within existing natural-gas fired combustion units such as boilers, furnaces, heaters, etc., as well as enabling automatic adjustment and operation, maximum usage of biogas, and integration with combustion unit controls. Using all available biogas to provide energy also minimizes the need for flaring unused biogas.