Leave the opamp with it's ground connected to the joint between the two resistors, choose the bias so it drives the output from the -2.5 V the opamp sees (which really is the 0 from the computer) to the +2.5V (which is the +5 of the computer).
I'm only giving the circuit a cursory look, but I thought he wanted a 9V reference voltage added to the sensor voltage going into R7. But I guess a lower reference voltage may be adequate too.
The midpoint voltage of the voltage divider used for the reference is 0.45V, well within the -2.5 to +2.5 swing.
The reason why I think it matters is that batteries are a nuisance, they run empty and will cause a problem with long running experiments, so it's worth the extra time to engineer them out of the circuit.
From what I can see they're only used to power the op-amp.
The 'floating ground' trick is s.o.p. when designing op-amp circuitry that needs to be fed from a single supply.
I said reference voltage, but I meant the voltage for the summing amp would need to be 9V if that's the step up range he's going for. I totally agree about eliminating the batteries if possible; I just wasn't sure how to get adequate voltage available to the amp without the 9V batteries. But again, it's been many years since I've even looked at a schematic. The floating ground idea looks extremely cool though.
Reading the post again he might be able to completely get rid of the whole circuit by placing two Ge diodes in series with the 0 terminal of the LM35, that will raise the 0 by 0.4 V, just enough to get over the threshold.
Leave the opamp with it's ground connected to the joint between the two resistors, choose the bias so it drives the output from the -2.5 V the opamp sees (which really is the 0 from the computer) to the +2.5V (which is the +5 of the computer).