You can't use nitrogen instead of CO2. It is used in conjunction with CO2. A 75% to 25% nitrogen to CO2 ratio, IIRC. You need to use CARBON dioxide to CARBONate.
In order to serve a stout through a stout faucet, which has a restrictor plate, a high serving pressure is required. If only CO2 was used it would be highly overcarbonated. Using a 75/25 mix of nitrogen and CO2 allows for this higher pressure without overcarbonating. When the beer passes through the restrictor plate in the faucet the pressure is reduced, the nitrogen is knocked out of solution and gives you that famous cascading head of tiny bubbles.