The condensation phenomena of the air-water mixture inside vertical mini-channels have been numerically investigated. The gap size of the channel was varied from 0.6 mm (2e-3 ft) to 1.2 mm (3.9e-3 ft), and the super-hydrophilic, hydrophilic, and hydrophobic surfaces were considered. The effect of the air velocity through the channel was also predicted. The liquid-vapor interface was captured using the volume of fluid (VOF) method, and Lee's phase-change model was adopted for condensation modeling. The numerical procedure was tuned and validated using previous experimental data. The film condensation was observed in the super-hydrophilic mini-gap channel, while the dropwise condensation and bridging of droplets occurred on other surfaces. Details of flow patterns and heat transfer characteristics were summarized in both quantitative and qualitative ways. The wettability and gap size with optimal heat transfer of condensation was estimated, which could provide guides to improve the condensation heat transfer in mini-channels.