TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Component: Water scrubber(s) is provided for removing contaminants from the feedstock gas emitted from the third filter vessel. Condenser(s) is provided between the second and third filter vessels. Water-gas shift reactor(s) is located between the second and third filter vessels.Preferred Method: Passing the feedstock gas at a first temperature to the first filter vessel comprises passing the feedstock gas at 700-1200 degrees F. Passing the feedstock gas at a second temperature that is less than the first temperature to the second filter vessel comprises passing the feedstock gas at a second temperature at 550-950 degrees F. Passing the feedstock gas at a third temperature that is less than the second temperature to third filter vessel(s) comprises passing the feedstock gas at a third temperature at 300-550 degrees F. The feedstock gas is passed through condenser(s) between the second filter vessel and the third filter vessel. It is heated to the third temperature after the feedstock gas exits the condenser and before the gas enters the third filter vessel. It is passed through scrubber(s). It is passed through a water-gas shift reactor, having an inlet temperature of 700 degrees F and an outlet temperature of 400 degrees F, located between the second filter vessel and the third filter vessel. It is passed through power recovery expander(s) between the second and third filter vessels. Removing a portion of particulates contained in the feedstock gas in the first filter vessel and removing a portion of halide(s) contained in the feedstock gas in the first filter vessel results in the feedstock gas having particulates less than 0.1 ppmw and a halide of less than 5 ppmv. Removing a portion of contaminants contained in the feedstock gas in the third filter vessel results in the feedstock gas having particulates less than 0.1 ppmw, less than 60 ppbv sulfur, less than 10 ppbv halide, less than 10 ppmv ammonia, less than 10 ppbv hydrogen cyanide, and less than 99% of a mercury content of a fuel source used to produce the feedstock gas. A feedstock gas from the third filter vessel having a concentration of carbon dioxide equal to an initial carbon dioxide concentration is emitted in the feedstock gas produced by the gasifier and having a concentration of water equal to an initial water concentration in the feedstock gas produced by the gasifier.