There's a 50/50 chance you'd even get high, let alone even feel the rush. Since you're on methadone. Granted, the dose varies from person to person on when the blocking properties of done are present. But even being on 50MG/Day could block you from feeling it. But if it doesn't block it, the feeling of the heroin will be greatly diminished. & odds are, you'll just be disappointed.
I mean, you've got NINE months sober, why throw that away for one night? & not to mention, your tolerance is still going to be higher, instead of lower, because of the done. So while you COULD feel your dose, at the same time, you could also need double or triple your dose, to breakthrough the done. I'm not saying to do that, not at all. But, that's what it COULD take.
Either way, if I were you, I would start out small. Adding another potent opiate on top of methadone could cause disaster, that's a LOT of CNS depression. But yeah, start small, then slowly work your way up... If you even feel anything at all.
Good luck, man. Stay safe.