Gas Heat Leaving Yellow Stains on Walls

My suggestion is to do the same thing as people used to do 50 years ago, 100 years ago and even more years ago than that; and that is to wash the walls (a simple vinegar and water or soap and water solution with a rag) every month or so. The cleaning was much worse way back then when it was wood or coal for heating than gas is so count yourself lucky. ;-)

Paint your wall yellow!