NOVELTY - A biomass material optionally added with inoculants, is inserted into a portion of a plastic bag (28). Both ends of biomass filled portion are sealed to form an unfilled portion into which methane gas is directed. A tube (30) is connected into the bag at the unfilled portion so that one end of the tube is protruded from the bag. The methane gas is directed through the tube to a point of collection or use.    USE - For producing methane gas from biomass waste material such as cattle ranches and dairy farms, and producing fertilizer as byproduct.    ADVANTAGE - The method is simple, and enables low cost production of methane gas by replacing fixed structures with a large flexible plastic bag. The flexible bags are available in varying sizes, and can be deployed from a bag filling machine as desired to accommodate the amount of biomass material to be processed. The bags are used to compost the biomass material upon depletion of methane production. The biomass material after composting can be used as fertilizer and the bags are readily collapsed to small fraction for disposal or recycling. The system for producing methane gas is portable and can be extended, contracted or totally collapsed to accommodate the need of the user, before or after completion of methane production.    DETAILED DESCRIPTION - A biomass material is optionally added with inoculants and inserted into a portion of a large flexible plastic bag. Both ends of biomass filled portion of plastic bag are sealed to form a unfilled portion with biomass material. The methane gas emitted from biomass material is directed to the unfilled portion of bag. A tube is connected into the bag at the unfilled portion so that one end of the tube is protruded from the bag. The methane gas is directed from the bag through the tube to a point of collection or use. An INDEPENDENT CLAIM is included for system for generating methane gas which has a flexible bag and a pipe. The flexible bag has a horizontally extended tubular length whose maximum length is filled with biomass material which produces methane gas and the remaining length is unfilled with the material so as to fill with the gas emitted by the biomass material.    DESCRIPTION OF DRAWING(S) - The figure shows flexible bag used for methane gas production.    Bag (28)    Tube (30)    Perforated conduit (34)