Abstract: A method of producing a synthetic gas or a synthesis gas from a biosolid, such as dewatered sludge, that has a solids content that does not exceed 30 wt. %. The biomass having a solids content that does not exceed 30 wt. % is mixed with tar-rich materials and/or char particles, and optionally a bulking agent. The tar-rich materials and/or char particles may be a by-product of producing synthesis gas from a biomass such as refuse-derived fuels. The resulting mixture then is heated to provide a mixture having a solids content of at least 75 wt. %. The mixture having a solids content of at least 75 wt. % then is gasified under conditions to produce a synthetic gas rich in CO/CO2 or a synthesis gas rich in H2/CO.