YOU SAID:

I've got to go away (but, baby, it's cold outside)

INTO JAPANESE

私は立ち去らなければなりません（でも、赤ちゃん、外は寒いです）

BACK INTO ENGLISH

I have to leave (but baby, cold outside)

INTO JAPANESE

私は出発しなければなりません（しかし、赤ちゃん、外は寒い）

BACK INTO ENGLISH

I have to leave (but baby, cold outside)