Yes, LIFELINE is a Scrabble word!

Letter values in Scrabble US: L 1, I 1, F 4, E 1, L 1, I 1, N 1, E 1.

Is it a valid word?

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

a crease on the palm; its length is said by palmists to indicate how long you will live
support that enables people to survive or to continue doing something (often by providing an essential connection)
line that raises or lowers a deep-sea diver
line thrown from a vessel that people can cling to in order to save themselves from drowning

Also: line of life, life line.