Activated carbon monoliths with developed porosity, high surface area and excellent adsorption properties were successfully prepared from resorcinol-formaldehyde resins using a physical activation method. The primary objective of this study was to examine the impact of key parameters, namely hexamethylenetetramine content (0.08–0.2 g), pyrolysis heating rate (5–20 °C/min) and activation time (1–7 h), on the final characteristics of the activated carbon in order to identify the optimal operating conditions to achieve the desired properties. All the cured resin samples were pyrolyzed at 900 °C under a nitrogen atmosphere, while the activation process took place in the presence of CO₂. The evaluation of the activated carbon materials was based on the CO₂ adsorption capacity and BET surface area, micropore area and total pore volume, which were employed as the criteria for selecting the optimal activated carbon. The synthesized porous carbon monoliths exhibited good properties: high BET surface area (900 m²/g), high CO₂ adsorption capacity (5.33 mmol/g at 0 °C and 1 bar, 3.8 mmol/g at 25 °C and 1 bar) and good CO₂ selectivity for CO₂/N₂ and CO₂/CH₄ mixtures. These results were obtained with a pyrolysis heating rate of 5 °C/min and a 3 h activation period.