Stuck valve on heating system
Heat is an option, just note that if the valve seats with any kind of polymer/rubber seals you'll likely destroy them.  In my experience, heat is never gentle if it's going to be effective.  

I'd go for heat first (like acetylene-hot, not that weak propane/mapp hot), but be VERY ready to have to go out and replace the whole valve.  

From looking at your picture, I'd cut above the 90 degree bend and sweat in a new bend and valve. After all, you're going to want to open and shut the valve at some point in the future, if you deal with it now, you'll be thankful later.