You didn't say what kind of art it is. If it is an open-edition, offset lithograph (like a poster, for example) then you might be better off to just buy a new one.
Some dry mount adhesives are releasable with re-application of heat (thermoplastic, as I recall), but some are not affected by re-heating (thermosetting, as I recall).
If the heat trick doesn't work, you could try adhesive release. In an open space with plenty of ventilation, place your problem on a flat table top. Pour a generous amount of adhesive release on the affected area and cover it with a piece of plain glass. The glass covering slows evaporation and helps the solution work. If it evaporates before the adhesive is dissolved, do it again.
That will definately dissolve the adhesive, and it won't hurt most lithographs. However, if you breathe the fumes you will die early. Be careful with toxic things.
If the print is old and dirty, you would see soil-tidelines afer the solution evaporates.
And the thickness of the tissue might leave a debossed mark on the art paper, where it was pressed.
If the art is anything other than an open-edition lithograph, you probably shouldn't have mounted it in the first place. If that's the case and you're uncertain about how to solve the problem, call a conservator.
Please don't ask how I know this stuff.