We carried out two-dimensional numerical simulation for downward Poiseulle water flow with a carbon-dioxide bubble or multiple carbon-dioxide bubbles in a vertical channel. A phase-field method was used for capturing the interface. The concentration equation including the phase-field function and the time evolution equation of the phase-field function including the velocity field were solved simultaneously by using Graphics Processing Units. The results showed that a bubble in the offset position moved away from the left-hand side wall, while it was rising, due to the lift force. This motion causes the development of asymmetrical, complicated structure of wake flow. Then, the bubble moved towards the wall due to the change in the flow velocity field. These time changes in the wake flow enhance the diffusion and absorption of carbon dioxide in flow. In the case of four bubbles, the decreasing rate of bubble mass is higher than that for a single bubble. This is partly because of the difference in the interface length, and partly because of the renewals both of the concentration boundary layer and the wake-flow structure caused by the impact of bubble coalescence.