I could smell gas so I removed the gas cap and there was a lot of pressure in the tank. so much that gas mist came out with hot vapor. what can be the problem.

Modern fuel systems are closed loops...tanks are unvented and can build up positive pressure due to high ambient heat. During highway travel several factors can contribute to the add heat. Road material (dark, heat absorbent concrete or asphalt), exhaust system heat, and the increase in temperature during the day while you drive. The release of pressure from the gas cap is a good sign the tank and its hoses are intact. Now, you have a fuel smell t to track down. Get your sniffer out and try to locate the source while the car is idling. If you can't find it, take it to a mechanic for a thorough inspection...they have better sniffing tools.

there is a valve to relive this pressure in modern fuel systems, normally it is located near the intake or the charcoal tank. it could be damaged or clogged. you need to check this valve and the hose. this valve is normally controlled by the ecu, check the codes to see if it has something related to this matter.