If you are dealing in ppm levels in water you might be better off using a colorimetric method for the formaldehyde. Acetylacetone is cheap, fast and very, very sensitive, so you can dilute your samples (working range is 1-500 ug/L, or 0.001 to 0.005 ppm). Methanol is still better by GC-FID, so you would analyze it by GC-FID and the formaldehyde colorimetrically. 

Methanol can be determined by Gas chromatography with headspace but formaldehyde is not possible by GCFID. So, you must try formaldehyde derivatisation by 2,4-DNP, it will show good peak on HPLC.

Depends on your concentration. Formaldehyde does not have a high response factor using FID. If you are dealing with part-per-million concentrations then you can use GC-FID; if you are looking for part-per-billion concentrations you will never see it.

Of course you are aware that Restek publishes chromatographic conditions for this column, right? When you ask us for conditions that are published it makes it look like you have just not done your homework.

As noted above by Ravi and Mark, you will not be able to measure both methanol and formaldehyde on the same FID GC unless the formaldehyde content is very high. Normally a TCD (katharometer) is used for this work. Alternatively you can add a methanizer before the FID, this is a small heated nickel catalyst bed that reacts organic materials with hydrogen to reduce them to methane. This will work for low sample concentrations with your existing FID. Here are links from ARC, Agilent, and Restek.

http://www.activatedresearch.com/technology/

https://www.agilent.com/cs/library/usermanuals/Public/G2747A%20Nickel%20Catalyst_030074.pdf

http://www.restek.com/pdfs/203-04-003.pdf

Thank you sir very much for your kind recommendations. Ours have a GC equipped with FID and TCD. Formaldehyde we are dealing with ppm. Actually I need to determine the concenrration of formaldehyde and methanol in water. I am struggling with preparing calibration samples. Can you please suggest me about the calibration samples that I can use? Should I use different concentrated Formaldehyde and Methanol? I did not find standatd reference material for those. Thanks 

You can make up ppm solutions by weighing out the pure reagents and then use serial dilutions to get the concentrations you want. For formaldehyde weigh 0.100gm of 95%+ pure reagent grade paraformaldehyde in 25mL of DI water and warm and stir/shake until it dissolves completely. At this low level you should not need to add NaOH to dissolve the paraformaldehyde.  Then wash the solution into a 100mL graduated flask and make up to the graduation with DI. This will give you a 950+ ppm solution. Diluting 10.0mL to 100mL with give you 95+ ppm etc. Do something similar with methanol - 1.0mL of 99.8% pure methanol in 100mL will give you 7,904 ppm, diluting 1mL of this to 100mL gives you 790 ppm, then 10mL of that solution into 100mL of water to give 79.0 ppm, etc. You will need to make the standards up fresh every few days, and keep them in the fridge when not in use. You could mix the dissolved standards 1:1 (a combined solution of 475/395 ppm) so that you need only one injection/dilution. Good lab practice implies that you should calibrate with 4 or 5 different concentration levels spread across your expected range. This is basic chemistry...

https://www.sigmaaldrich.com/catalog/product/sial/p6148?

If you are dealing in ppm levels in water you might be better off using a colorimetric method for the formaldehyde. Acetylacetone is cheap, fast and very, very sensitive, so you can dilute your samples (working range is 1-500 ug/L, or 0.001 to 0.005 ppm). Methanol is still better by GC-FID, so you would analyze it by GC-FID and the formaldehyde colorimetrically.