A gas cleaning system for removing at least a portion of contaminants, such as halides, sulfur, particulates, mercury, and others, from a synthesis gas (syngas). The gas cleaning system may include one or more filter vessels coupled in series for removing halides, particulates, and sulfur from the syngas. The gas cleaning system may be operated by receiving gas at a first temperature and pressure and dropping the temperature of the syngas as the gas flows through the system. The gas cleaning system may be used for an application requiring clean syngas, such as, but not limited to, fuel cell power generation, IGCC power generation, and chemical synthesis.