A method for providing upgraded biogas includes feeding a stream of biogas into a biogas upgrading system in order to remove carbon dioxide from the stream of biogas. The biogas upgrading system, which may be based on absorption, adsorption, membrane permeation, and/or cryogenics, provides a stream of upgraded biogas and a tail gas stream. The tail gas stream, which may be CO2-rich, is enriched with natural gas so that it is combustible in medium-BTU equipment. The upgraded biogas is used for transportation use and/or the generation of fuel credits. Accordingly, both the tail gas and the upgraded biogas are used effectively and at lower cost.