Why does the Torah write again that Chanoch went with Hashem, and then adds "and he was no more, for Hashem took him"?
Rashi: To teach us that although he was a Tzadik, Hashem took his life prematurely, because he was prone to become a Rasha. 1
Targum Yonasan: He lived three hundred and sixty-five years here in this world, before Hashem took him away and made him an angel - Matatron the Great Scribe. 2
Moshav Zekenim (1): He was the seventh generation from Adam. The seventh is always dear - Moshe was seventh from Avraham and he went up to Hashem, and Eliyahu was seventh. 3
Moshav Zekenim (2): It says "he was not" for he lived much less than the others.
Like we find with regard to a ben Sorer u'Moreh, about whom Chazal said 'it is better that he die innocent than guilty'.This also explains why he died so much younger than any of the other people listed in this Parshah.
See also Rosh, Ba'al ha'Turim.
It is not clear how Eliyahu was seventh. Bava Basra 121b lists seven whose lives spanned the world; Eliyahu is the seventh. Perhaps the text should that David was seventh son, stating from Eli'av, like it says in Yalkut Shimoni (Yisro 276) (PF).