Btu values are specific to the particular fuel. Expressed in Btu per unit mass, this does not change with heat or pressure. On the other hand, if it is expressed in Btu per unit volume, then it will change with changes in temperature and pressure.
__________________
"Reason is not automatic. Those who deny it cannot be conquered by it. Do not count on them. Leave them alone." - Ayn Rand
The original poster withheld the flowrate, temperature and pressure, as well as the composition of the <...gas...> in question. It doesn't give much for the forum to go on, does it?
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Good Answers: