In an integrated gasification combined cycle (IGCC) power plant, the initial operating procedure is driven by natural gas first, and then by H₂/CO syngas. This study attempted to find the optimal operating procedure in the fuel changeover section, such as from natural gas to syngas. Therefore, the influences of various swirl combinations and syngas compositions on flame structures and NO_x and CO emissions characteristics were investigated through experimental and numerical analyses in a partially premixed dual swirl combustor. To observe the various flame structures, such as lifted, tubular, and attached flames, the two-dimensional particle image velocimetry (PIV) technique was applied and compared with the results obtained by computational fluid dynamics (CFD). The combustion characteristics of co– and counter-swirling flames were observed, and the optimal swirl combination was subsequently confirmed by using the parameter of modified unmixedness. When the flame shape and fuel and air mixing efficiency are considered, S_{co, 1.3/1.46} condition is the optimal swirl combination for stable conversion of the syngas fuel. In addition, the operating parts with a methane content of 25% and a hydrogen content of 75% are considered to be avoided in the operating part of fuel changeover. The operating part of fuel changeover can be divided into 10 paths for stable syngas operation. The optimal operating procedure to control the content of hydrogen and carbon monoxide after the syngas operation with the Path No. 2, such as H₂ = 25% and CO = 75%.