Yes, MAINFRAMES is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, I 1, N 1, F 4, R 1, A 1, M 3, E 1, S 1.

Is it a valid word?

MAINFRAMES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a large digital computer serving 100-400 users and occupying a special air-conditioned room
(computer science) the part of a computer (a microprocessor chip) that does most of the data processing

Also: mainframe computer, central processing unit, c.p.u., central processor.

Not playable in Scrabble US: CPU.