Interfacing LM35 and MQ2 with Arduino
A wiring diagram would help! But it sounds like you're powering your sensor from the Arduino's AREF pin. That is not the purpose of AREF; AREF is there for you to provide an accurate +5v or other reference voltage to the A/D convertor.
You need to power the sensor independently from your 5v supply (and not through the Arduino either, because of the sensors current draw); connect the sensor's Analog Out to one of the Arduino's A/D pins, sensor Ground to the supply ground. The digital out appears to be optional, if all you want is a go/no-go output that you adjust with the on-sensor potentiometer.