TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The gas emitted from syngas cooler (14) is introduced into syngas cooler (17), and cooled to 40degreesC or less by exchanging heat with a cooling water. The light oil contained in gas is condensed and removed, while condensing the steam. The gas emitted from syngas cooler (17) is introduced into an electrostatic precipitator (18) to remove dust, mist-like tar, light oil, and water droplet remaining in the gas. The gas cooled with syngas cooler (17) is heated and the temperature of the gas in the path to a gas supply place, is maintained at more than the condensation temperature of the tar contained in gas. The water phase of syngas cooler (14) and syngas cooler (17) is made to absorb acidic gas, such as hydrochloride and hydrogen sulfide, aqueous solution such as sodium hydroxide and ammoniacal liquor (NH3), is poured into water phase as a neutralizing agent and the acidic gas of the gas side is removed. The pressure of gas emitted from the electrostatic precipitator is increased by heat exchange or gas exhauster (19) with indirect heat exchanger (4) using steam, and passed through activated carbon filled layer or activated carbon moving bed (21) filled with active coke or granular active carbon. Incineration processing is carried out with the combustion furnace (16) which supplies heat for waste water generated from syngas cooler (14), syngas cooler (17), and thermal decomposition furnace to thermal decomposition furnace or combustion furnace (3) which combusts the combustion furnace waste gas. Solid substance and tar of the wastewater generated from syngas cooler (14), syngas cooler (17), and electrostatic precipitator are separated by pressurization floating of light oil or by specific gravity difference. Alternately, alkali is added to water layer after cooling wastewater, and adjusting pH to 9.5-12, separation is carried out by coagulating-and-sedimenting the metal contained in the wastewater. Alternately, nitrogen and ammonia are removed by ammonia stripping. The byproduct ammoniacal liquor obtained by coke-oven-gas refinement process of iron mill is uses as a neutralizing agent for supplementary water of syngas cooler (14) and syngas cooler (17).