MOF's can be considered to be somewhat flexible structures sometimes. This flexibilty, results CO2 presence to create even more space than before. Hence, MC only simulations may not be enought to simulate what happens in real life, instead MD simulations inbetween are also needed.

All of this, of course assumes the MOF-CO2 interactions are all set correctly.

Thanks Marcel Balçık for your answer.

However, I have checked MC adsorption of other gases such as hydrogen sulphide, water, Methane and Ethan in same MOF and I have got well matched with experimental measurements. But for MC adsorption of carbon dioxide is lower than experimental.   

Yes, and thats probably because CO2 is the only gas among them that cause swelling.