What's the least painful way to get rid of rats in house?

Seal the gaps first, otherwise you're just killing the ones already in while more come through. Rats get in through surprisingly small holes - check where pipes come through walls, round the back of the kitchen units, vents, the garage door. Steel wool plus filler or metal mesh, they chew through foam. Snap traps baited with peanut butter along the walls (they run along edges, not across rooms). Clear food sources - pet food, bird feeders, compost. If you're hearing a lot of them or they're in the loft, get pest control in.